Tests of water off the US West Coast have found no signs of radiation from Japan's 2011 Fukushima nuclear disaster, although low levels of radiation are ultimately expected to reach the US shore.

Results obtained this week in tests of water gathered by an Oregon conservation group and tested by East Coast scientists came in as expected with no Fukushima-linked radiation, and five more tests are planned at six-month intervals to see if radiation levels will climb.

"We've seen radiation halfway across the Pacific, north of Hawaii, but in US waters there has been none, yet," Woods Hole Oceanographic Institution senior scientist Ken Buesseler said.

Tests of some fish species, which can race across the ocean more quickly than slow-moving currents, have shown higher levels of radiation, although radiation levels in sea life off the US shore are still safe, Buesseler said.

York Johnson, water quality coordinator with Oregon conservation group Tillamook Estuaries Partnership, which collected the water samples, said scientific models of ocean currents suggest that cesium 134 isotopes from Fukushima could reach Oregon's shores later this year in low levels.

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His organization is one of dozens that are essentially crowd-funding the study of ocean radiation after the federal government opted not to track the spread of cesium 134 from Fukushima toward the United States.

"We wanted to be able to talk to the community about what we are seeing - and to have evidence," Johnson said, adding that the coast should still be safe for recreation even if radiation levels climb over the next several years as expected.

Tests off the coast of Japan shortly after the 2011 earthquake and nuclear plant disaster measured radiation at 50 million becquerel per cubic meter, Buesseler said. A "becquerel" is a unit of radioactivity.

"When it is at 50 million, you have direct effects on mortality, reproductive effects. That is a seriously high number," Buesseler said. "The U.S. Environmental Protection Agency allows us to drink water up to about 7,400."

He predicts that water off the Oregon shore will peak at around 10 to 20 becquerel per cubic meter of water as ocean currents carry radiation from Japan in the years ahead.